Hi Richard,
OK, First, I assume you've downloaded and installed Microsoft Virtual PC? http://www.microsoft.com/windows/virtual-pc/download.aspx
If you have, look in your start menu and you'll have a Windows Virtual PC option, go into that menu and click on Windows Virtual PC.
Now, there is technically no GUI for Virtual PC, it's integrated into Windows 7 so it just looks like a Windows Explorer menu, but it's not.
If you look towards the top of the Window, there is a Create Virtual Machine button, click that and it will guide you through creating a new virtual machine.
When it gets to the part of asking you about your hard drive (3rd screen I think), opt to select a VHD file and choose the VHD file you created. Complete the wizard and start the machine.
That's it -
Let me know if you still stuggle.
Regards,
Jon